Wireless mobile communication technology uses various standards and protocols to transmit data between a node (e.g., a transmission station) and a wireless device (e.g., a mobile device). Some wireless devices communicate using orthogonal frequency-division multiple access (OFDMA) in a downlink (DL) transmission and single carrier frequency division multiple access (SC-FDMA) in an uplink (UL) transmission. Standards and protocols that use orthogonal frequency-division multiplexing (OFDM) for signal transmission include the third generation partnership project (3GPP) long term evolution (LTE) and LTE-Advanced (LTE-A).
In 3GPP radio access network (RAN) LTE and LTE-A systems, the node can be a combination of Evolved Universal Terrestrial Radio Access Network (E-UTRAN) Node Bs (also commonly denoted as evolved Node Bs, enhanced Node Bs, eNodeBs, or eNBs) and Radio Network Controllers (RNCs), which communicate with the wireless device, known as a user equipment (UE). Examples of a UE include a mobile terminal, a tablet computer, a personal digital assistant (PDA) and a machine-type communication (MTC) device. A downlink (DL) transmission can be a communication from the node (or eNodeB) to the wireless device (or UE), and an uplink (UL) transmission can be a communication from the wireless device to the node. Instead of communication via eNodeBs, communication between wireless equipment can be performed using peer-to-peer or device-to-device communication. A wireless equipment includes at least a UE, a picocell, a femtocell and a relay node.
As mobile technology advances, there is a requirement to provide increasing peak data throughput to meet user demand, for example to provide high data-rate services. Some services, such as Voice over Internet Protocol (VoIP) are delay sensitive and should respect Quality of Service (QoS) constraints.
Carrier aggregation allows a single wireless connection to use multiple radio frequency (RF) carriers, known as Component Carriers (CCs) and increases channel bandwidth so that peak and average throughput can be increased. LTE Release 10 version defines signaling to support up to five component carriers to give a maximum combined channel bandwidth of up to 100 MHz. Component carriers can be intra-band contiguous, intra-band non-contiguous or even located in different bands (inter-band non-contiguous). Carrier aggregation is applicable to both uplink and downlink directions and to both Frequency Division Duplex (FDD) and Time Division Duplex (TDD).
Carrier aggregation is considered as one of the main approaches to increase data rate performance of LTE-A systems and beyond. There are a large number of Release-10 UEs already available on the market that support aggregation of two carriers. It is expected that demands of carrier aggregation (CA) capable UEs with aggregation of multiple carriers in the future will become even higher. For example, LTE with licensed-assisted access (also known as LTE for unlicensed spectrum) may operate a with large number component carriers that may be aggregated at the UE to increase the peak data rate.
Carrier aggregation categorizes cells as primary cells and secondary cells. The primary cell is the cell upon which the UE performs an initial connection establishment and each wireless connection has a single primary cell that uses a primary radio frequency (RF) carrier that can be changed during a handover procedure. One or more secondary cells are configured after connection establishment to provide additional radio resources on a respective secondary RF carrier. Although the RF carriers are typically licensed carriers (e.g. LTE or LTE-A), as mentioned above the secondary RF carriers could use unlicensed RF spectrum for supplemental downlink and/or uplink capacity. Examples of unlicensed frequencies that could be utilized for carrier aggregation are 5 GHz, 2.4 GHz and 5150-5350 MHz.
Automatic Repeat reQest (ARQ) is a retransmission protocol where the receiver checks for errors in the received data and upon detection of an error, discards the data and requests retransmission from the sender. Hybrid ARQ (HARQ) is a retransmission protocol in which, if an error in received data is detected, the receiver buffers the data and requests retransmission from the sender. An HARQ receiver improves performance of the retransmissions by combining the re-transmitted data with the buffered data prior to channel decoding and error detection. The Media Access Control (MAC) layer uses an HARQ protocol. In the case of carrier aggregation, an HARQ entity is required for each of the plurality of configured carriers of the aggregated carrier. A “soft buffer” is required to buffer encoded data (“soft bits”) of transport blocks in which the receiver has detected an error, so that the data combining can be performed to improve decoding efficiency. As the number of component carriers increases, the corresponding increasing requirement for soft-buffer capacity can become problematic.